Ground Water Site Condition Score

And Exceedance Quotient

March 2012

Remedial Priority System

1

3 6

2

7 4

Definitions: Exceedance Quotient vs Site

Condition Scores

• An exceedance quotient (EQ) is a normalized value that enables SRP to compare analytes with different properties and different concentrations across distance and time. Each analyte that is sampled will be converted into an EQ.

• An Site Condition Score is one value that represents the site and is calculated using the Exceedance Quotients.

GW Site Condition Scores

• The Site Condition Score is created using the electronic sampling results submitted to the Department and one value is produced for each media evaluated by the RPS Model.

• At this time, there are three Media evaluated as part of the RPS Model.

• Ground Water

• Soil

• Vapor

All results between 8/4/2007 and 1/20/2010 would be used to calculate the ground water Site Condition Score

Calculate the GW Site Condition Score

What data are used to calculate the GW Site Condition Score:

– The RPS only uses the latest sampling round and all other events within 900 days of that sampling event to determine the GW Site Condition Score

– Reasons for 900 days: • Limit the time of sample reviews to ensure only most recent samples

are being considered in calculations

• 900 days would consist of 10 rounds of quarterly monitoring

1/20/2010

8/4/2007

900 days

Example: Last Sample Round was 1/20/2010

• An exceedance quotient (EQ) is calculated for

each ground water result that exceeds the Ground Water Quality Standards (GWQS)

Calculate the GW Site Condition Score

Adjustments to the initial concentration

• Solubility: – Water Solubility (S) is “… the equilibrium distribution of a solute between

water and solute phases at a given temperature and pressure. Because S is the maximum solute concentration possible at equilibrium, it can function as a limiting factor in concentration-dependent processes." - Pontolillo and Eganhouse, 2001 from USGS Toxic Substances

• Mobility: – Retardation factor (R) *1 is the ratio of the groundwater seepage velocity to

the rate that organic chemicals migrate in the ground water.

• Degradation: – Half-life (t1/2)

*1 is the period of time required for an analyte to

decrease by half. *1 a retardation and half-life are calculated for each compound based on typical aquifer characteristics

Calculate the GW EQ

Exceedance quotient Calculation

• Health based Ground Water Quality Criteria: – Ground Water Quality Criteria (GWQC)

• Natural Log: – Ground water contamination at a site has a log-normal distribution. The

natural log is used to normalize the value.

Calculate the GW EQ

Adjustments to the initial concentration

Exceedance quotient Calculation

Statistical Calculation Flowchart

GWQC = Ground Water Quality Criteria

R = Retardation Factor

S = Solubility

t½ = Half Life (years)

Legend

Method to Calculate GW EQ

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

Calculate GW EQ

Start with the analyte concentration (if it exceeds the appropriate GWQS in a well)

Compare the concentration to the analyte’s solubility

– If the solubility is lower than the concentration, then substitute the solubility for the concentration.

– If the solubility is greater than the concentration, then use the analyte concentration.

1. Adjust for Solubility

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

GW EQ Steps

2. Compare to GWQC

Divide the value that was calculated from the last step by the health based Ground Water Quality Criteria (GWQC).

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

GW EQ Steps

3. Adjust for Mobility

Evaluate the Retardation Factor for the compound.

– If the Retardation Factor is greater than 2 for the compound, then divide by 2

– If the retardation Factor is less than or equal to 2 for the compound, then divide by 1.

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

GW EQ Steps

4. Adjust for degradation

Evaluate the half life of the analyte.

– If the half life for the analyte is less than 2 years, then divide by 2.

– If the half life for the analyte is greater than or equal to 2 years, then divide by 1.

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

GW EQ Steps

5. Natural Log

Calculate the natural log of the previous value.

– The natural log is used to normalize the value. Ground water contamination usually has a log normal distribution in nature. This step is used to normalize the results.

The result is the EQ for that sample result

t½ > 2 t½ < 2

Conc > S Conc < S

EQ

R > 2

Adjust for

Solubility

analyte

Concentration

Solubility Concentration

R < 2 Adjust for

Mobility divide by 1 divide by 2

Adjust for

degradation divide by 1 divide by 2

divide by GWQC

natural log

• The process to calculate a Site Condition Score for Ground Water includes 2 steps:

• Step 1

– calculate the average of the EQ for each analyte for each well

• Step 2

– calculate a 95% upper confidence limit (UCL) for the average of the EQs calculated in Step 1

• If there is an exceedance for an analyte in a well, then all of the results during the 900 day period will be used to evaluate that analyte.

Notes:

Calculate the GW Site Condition Score

Calculate the GW EQ • Calculate the EQ using solubility, mobility, degradation, and other

characteristics that affect travel time and distance.

Example Dataset:

Well

Analyte

Test

Conc

(ppb)

Evaluate Solubility

Value Result

If S>Conc then Conc

If S>Conc then S

Evaluate GWQC

Value Result

Divide by GWQC

Evaluate Mobility

Value Result

If R>2 then X 0.5

If R<2 then X 1

Evaluate Degradation

Value Result

If t1/2<2 then X 0.5

If t1/2>2 then X 1

Natural

Log

EQ

MW-1 Benzene 5,000 1,780,000 5,000 .2 25,000 R < 2 25,000 T ½ < 2 12,500 9.43 9.43

MW-1 Toluene 5,000 535,000 5,000 600 8.33 R > 2 4.16 T ½ < 2 2.08 0.73 0.73

MW-1 Xylenes 50,000 175,000 50,000 1000 50 R > 2 25 T ½ < 2 12.5 2.53 2.53

MW-2 TCE 20 1,100,000 20 1 20 R > 2 10 T ½ > 2 10 2.30 2.30

MW-4 PCE 1,500 200,000 1,500 0.4 3,750 R < 2 3,750 T ½ > 2 3,750 8.23 8.23

• The process to calculate a Site Condition Score for Ground Water includes 2 steps:

• Step 1

– calculate an average EQ for each analyte for each well

• Step 2

– calculate a 95% upper confidence limit (UCL) for the average of the EQs calculated in Step 1

• Calculate a 95% UCL of the averages previously calculated for the rounds of wells and analytes.

• If there are less than 4 samples, the mean will be used. • If there are 4 or more samples, then the 95% Upper Confidence Level will be

used.

Notes:

Calculate the GW Site Condition Score

analyte Well EQs

Benzene MW-1 9.43

Toluene MW-1 0.73

Xylenes MW-1 2.53

TCE MW-2 2.30

PCE MW-4 8.23

Sample

Count Average

Standard

Deviation

Significant Interval

95% UCL

5 4.64 3.91 3.42 8.07

Ground Water SCS = 8.07

The Site Condition Score for Ground Water is the 95% UCL

Calculate the GW Site Condition Score

Calculate a 95% UCL of all the UCLs previously calculated for the rounds of wells and contaminants. There is now one UCL for the entire site.

Example Dataset

• Assorted Notes:

• If a Site Condition Score is negative, 0.001 is substituted for the final Site Condition Score. The replacement of the negative value is needed to make sure that all of the numbers are positive.

• Negative EQ values are used to Calculate the 95% UCL.

Calculate the GW Site Condition Score